The present invention is directed to a method of making carboxylated cellulose fibers including those in which fiber strength and degree of polymerization is not significantly sacrificed. The invention is further directed to the carboxylated fibers and to products made using the fibers.
Cellulose is a carbohydrate consisting of a long chain of glucose units, all xcex2-linked through the 1xe2x80x2-4 positions. Native plant cellulose molecules may have upwards of 2200 anhydroglucose units. The number of units is normally referred to as degree of polymerization or simply D.P. Some loss of D.P. inevitably occurs during purification. A D.P. approaching 2000 is usually found only in purified cotton linters. Wood derived celluloses rarely exceed a D.P. of about 1700. The structure of cellulose can be represented as follows: 
Chemical derivatives of cellulose have been commercially important for almost a century and a half. Nitrocellulose plasticized with camphor was the first synthetic plastic and has been in use since 1868. A number of cellulose ether and ester derivatives are presently commercially available and find wide use in many fields of commerce. Virtually all cellulose derivatives take advantage of the reactivity of the three available hydroxyl groups. Substitution at these groups can vary from very low; e.g. about 0.01 to a maximum 3.0. Among important cellulose derivatives are cellulose acetate, used in fibers and transparent films; nitrocellulose, widely used in lacquers and gun powder; ethyl cellulose, widely used in impact resistant tool handles; methyl cellulose, hydroxyethyl, hydroxypropyl, and sodium carboxymethyl cellulose, water soluble ethers widely used in detergents, as thickeners in foodstuffs, and in papermaking.
Cellulose itself has been modified for various purposes. Cellulose fibers are naturally anionic in nature as are many papermaking additives. A cationic cellulose is described in Harding et al. U.S. Pat. No. 4,505,775. This has greater affinity for anionic papermaking additives such as fillers and pigments and is particularly receptive to acid and anionic dyes. Jewell et al., in U.S. Pat. No. 5,667,637, teach a low degree of substitution (D.S.) carboxyethyl cellulose which, along with a cationic resin, improves the wet to dry tensile and burst ratios when used as a papermaking additive. Westland, in U.S. Pat. No. 5,755,828 describes a method for increasing the strength of articles made from cross linked cellulose fibers having free carboxylic acid groups obtained by covalently coupling a polycarboxylic acid to the fibers.
For some purposes cellulose has been oxidized to make it more anionic; e.g., to improve compatibility with cationic papermaking additives and dyes. Various oxidation treatments have been used. Among these are nitrogen dioxide and periodate oxidation coupled with resin treatment of cotton fabrics for improvement in crease recovery as suggested by R. T. Shet and A. M. Nabani, Textile Research Journal November 1981: 740-744. Earlier work by K. V. Datye and G. M. Nabar, Textile Research Journal, July 1963: 500-510, describes oxidation by metaperiodates and dichromic acid followed by treatment with chlorous acid for 72 hours or 0.05 M sodium borohydride for 24 hours. Copper number was greatly reduced by borohydride treatment and less so by chlorous acid. Carboxyl content was slightly reduced by borohydride and significantly increased by chlorous acid. The products were subsequently reacted with formaldehyde. P. Luner et al., Tappi 50(3): 117-120 (1967) oxidized southern pine kraft springwood and summer wood fibers with potassium dichromate in oxalic acid. Handsheets made with the fibers showed improved wet strength believed due to aldehyde groups. P. Luner et al., in Tappi 50(5): 227-230 (1967) expanded this earlier work and further oxidized some of the pulps with chlorite or reduced them with sodium borohydride. Handsheets from the pulps treated with the reducing agent showed improved sheet properties over those not so treated. R. A. Young, Wood and Fiber, 10(2): 112-119 (1978) describes oxidation primarily by dichromate in oxalic acid to introduce aldehyde groups in sulfite pulps for wet strength improvement in papers. V. A. Shenai and A. S. Narkhede, Textile Dyer and Printer May 20, 1987: 17-22 describe the accelerated reaction of hypochlorite oxidation of cotton yarns in the presence of physically deposited cobalt sulfide. The authors note that partial oxidation has been studied for the past hundred years in conjunction with efforts to prevent degradation during bleaching. They also discuss in some detail the use of 0.1 M sodium borohydride as a reducing agent following oxidation. The treatment was described as a useful method of characterizing the types of reducing groups as well as acidic groups formed during oxidation. The borohydride treatment noticeably reduced copper number of the oxidized cellulose. Copper number gives an estimate of the reducing groups such as aldehydes present on the cellulose. Borohydride treatment also reduced alkali solubility of the oxidized product but this may have been related to an approximate 40% reduction in carboxyl content of the samples. R. Andersson et al. in Carbohydrate Research 206: 340-346 (1990) teach oxidation of cellulose with sodium nitrite in orthophosphoric acid and describe nuclear magnetic resonance elucidation of the reaction products.
N. J. Davis and S. L. Flitsch, Tetrahedron Letters 34(7): 1181-1184 (1993) describe the use and reaction mechanism of 2,2,6,6-tetramethylpiperidinyloxy free radical (TEMPO) with sodium hypochlorite to achieve selective oxidation of primary hydroxyl groups of monosaccharides. Following the Davis et al. paper this route to carboxylation then began to be more widely explored. A. E. J. de Nooy et al., in a short paper in Receuil des Travaux Chimiques des Pays-Bas 113: 165-166 (1994), report similar results using TEMPO and hypobromite for oxidation of primary alcohol groups in potato starch and inulin. The following year, these same authors in Carbohydrate Research 269: 89-98 (1995) report highly selective oxidation of primary alcohol groups in water soluble glucans using TEMPO and a hypochlorite/bromide oxidant.
PCT published patent application WO 95/07303 (Besemer et al.) describes a method of oxidizing water soluble carbohydrates having a primary alcohol group, using TEMPO with sodium hypochlorite and sodium bromide. Cellulose is mentioned in passing in the background although the examples are principally limited to starches. The method is said to selectively oxidize the primary alcohol at C-6 to carboxyl. None of the products studied were fibrous in nature.
PCT application WO 99/23117 (Viikari et al.) teaches oxidation using TEMPO in combination with the enzyme laccase or other enzymes along with air or oxygen as the effective oxidizing agents of cellulose fibers, including kraft pine pulps.
A year following the above noted Besemer publication, the same authors, in Cellulose Derivatives, T. J. Heinze and W. G. Glasser, eds., Ch. 5, pp 73-82 (1996), describe methods for selective oxidation of cellulose to 2,3-dicarboxy cellulose and 6-carboxy cellulose using various oxidants. Among the oxidants used were a periodate/chlorite/hydrogen peroxide system, oxidation in phosphoric acid with sodium nitrate/nitrite, and with TEMPO and a hypochlorite/bromide primary oxidant. Results with the TEMPO system were poorly reproduced and equivocal. The statement that xe2x80x9c. . . some of the material remains undissolvedxe2x80x9d was puzzling. In the case of TEMPO oxidation of cellulose, little or none would have been expected to go into solution. The homogeneous solution of cellulose in phosphoric acid used for the sodium nitrate/sodium nitrite oxidation was later treated with sodium borohydride to remove any carbonyl function present.
P.-S. Chang and J. F. Robyt, Journal of Carbohydrate Chemistry 15(7): 819-830 (1996),describe oxidation of ten polysaccharides including xcex1-cellulose at 0xc2x0 C. and 25xc2x0 C. using TEMPO with sodium hypochlorite and sodium bromide. Ethanol addition was used to quench the oxidation reaction. The resulting oxidized xcex1-cellulose had a water solubility of 9.4%. The authors did not further describe the nature of the xcex1-cellulose. It is presumed to have been a so-called dissolving pulp or cotton linter cellulose. D. Barzyk et al., in Transactions of the 11th Fundamental Research Symposium, Vol. 2, 893-907 (1997), note that carboxyl groups on cellulose fibers increase swelling and impact flexibility, bonded area and strength. They designed experiments to increase surface carboxylation of fibers. However, they ruled out oxidation to avoid fiber degradation and chose to form carboxymethyl cellulose in an isopropanol/methanol system.
Isogai, A. and Y. Kato, in Cellulose 5: 153-164 (1998) describe treatment of several native, mercerized, and regenerated celluloses with TEMPO to obtain water soluble and insoluble polyglucuronic acids. They note that the water soluble products had almost 100% carboxyl substitution at the C-6 site. They further note that oxidation proceeds heterogeneously at the more accessible regions on solid cellulose.
Kitaoka, T., A., A. Isogai, and F. Onabe, in Nordic Pulp and Paper Research Journal, 14(4): 279-284 (1999), describe the treatment of bleached hardwood kraft pulp using TEMPO oxidation. Increasing amounts of carboxyl content gave some improvement in dry tensile index, Young""s modulus and brightness, with decreases in elongation at breaking point and opacity. Other strength properties were unaffected. Retention of PAE-type wet strength resins was somewhat increased. The products described did not have any stabilization treatment after the TEMPO oxidation.
None of the previous workers have described a stable fibrous carboxylated cellulose that can be made in conventional papermill equipment in an aqueous system with minimum D.P. loss to yield a product with superior papermaking properties.
The present invention is directed to a fibrous carboxylated cellulose product, to the method of its manufacture, and to sheeted paper products using the carboxylated fibers. A chemically purified fibrous cellulose market pulp is the basic material for the process. This may be, but is not limited to, bleached or unbleached sulfite, kraft, or prehydrolyzed kraft hardwood or softwood pulps or mixtures of hardwood and softwood pulps. So-called high alpha cellulose or chemical pulps are not considered as raw materials included within the scope of the invention.
The suitability of lower cost market pulps is a significant advantage of the process. Market pulps are used for many products such as fine papers, diaper fluff, paper towels and tissues, etc. These pulps generally have about 86-88% xcex1-cellulose and 12-14% hemicellulose whereas the high xcex1-cellulose chemical or dissolving pulps have about 92-98% xcex1-cellulose. To the present inventors"" knowledge the lower xcex1-cellulose pulps or other cellulose having a high content of hemicellulose have never before been treated with TEMPO or other cyclic nitroxide compounds to produce a stable carboxylated fiber. By stable is meant minimum D.P. loss in alkaline environments, and very low self cross linking and color reversion. The method of the invention is particularly advantageous for treating secondary (or recycled) fibers. Bond strength of the sheeted carboxylated fibers is significantly improved over untreated recycled fiber.
The term xe2x80x9ccellulosexe2x80x9d when used hereafter and in the claims refers to a wood based cellulose market pulp below 90% xcex1-cellulose, generally having about 86-88% xcex1-cellulose and a hemicellulose content of about 12%.
The process of the invention will lead to a product having an increase in carboxyl substitution over the starting material of at least about 2 meq/100 g, preferably about 5 meq/100 g. Carboxylation occurs predominantly at the hydroxyl group on C-6 of the sugar units to yield uronic acids.
The literature has reported the use of TEMPO as an oxidant of primary hydroxyl group on various carbohydrate materials. However, there is little or no use of other nitroxide materials reported and the presumption has been that these were not effective oxidants. Surprisingly, the present inventors have found that a wide number of species of cyclic nitroxides are equally as effective as TEMPO and that many of them offer advantages in convenience of use, cost, or performance.
The cellulose fiber in an aqueous slurry or suspension is first oxidized by addition of a primary oxidizer comprising a cyclic nitroxide lacking any hydrogen substitution on either of the carbon atoms adjacent the nitroxide nitrogen. Nitroxides having both five and six membered rings have been found to be satisfactory. Both five and six membered rings may have either a methylene group or another heterocyclic atom selected from nitrogen, sulfur or oxygen at the four position in the ring, and both rings may have substituent groups at this location. It is important that the nitroxide chosen be stable in an aqueous alkaline environment in the range of about pH 8-11.
A large group of nitroxide compounds have been found to be suitable. 2,2,6,6-Tetramethylpiperidinyl-1-oxy free radical (TEMPO) is among the exemplary nitroxides found useful. Another suitable product linked in a mirror image relationship to TEMPO is 2,2,2xe2x80x22xe2x80x2,6,6,6xe2x80x2,6xe2x80x2-octamethyl-4,4xe2x80x2-bipiperidinyl-1,1xe2x80x2-dioxy di-free radical (BI-TEMPO). Similarly, 2,2,6,6-tetramethyl-4-hydroxypiperidinyl-1-oxy free radical; 2,2,6,6-tetramethyl-4-methoxypiperidinyl-1-oxy free radical; and 2,2,6,6-tetramethyl-4-benzyloxypiperidinyl-1-oxy free radical; 2,2,6,6-tetramethyl-4-aminopiperidinyl-1-oxy free radical; 2,2,6,6-tetramethyl-4-acetylaminopiperidinyl-1-oxy free radical; and 2,2,6,6-tetramethyl-4-piperidone-1-oxy free radical are examples of compounds with substitution at the 4 position of TEMPO that have been found to be very satisfactory oxidants. Among the nitroxides with a second hetero atom in the ring at the four position (relative to the nitrogen atom), 3,3,5,5-tetramethylmorpholine-1-oxy free radical (TEMMO) is very useful.
The nitroxides are not limited to those with saturated rings. One compound found to be a very effective oxidant is 3,4-dehydro-2,2,6,6-tetramethyl-piperidinyl-1-oxy free radical.
Six membered ring compounds with double substitution at the four position have been especially useful because of their relative ease of synthesis and lower cost. Exemplary among these are the ethylene, propylene, and neopentyl cyclic acetals of 2,2,6,6- tetramethyl-4-piperidone-1-oxy free radical.
Among the five membered ring products, 2,2,5,5-tetramethyl-pyrrolidinyl-1-oxy free radical has been found to be very effective.
The above named compounds should only be considered as exemplary among the many representatives of the nitroxides suitable for use with the invention and those named are not intended to be limiting in any way.
It is also considered to be within the scope of the invention to form the nitroxides in situ by oxidation of the hydroxylamines of any of the nitroxide free radical products. Oxammonium salts of the nitroxides are also suitable. While the nitroxide is consumed and converted to an oxammonium salt then to a hydroxylamine during the oxidation reaction, it is continuously regenerated by the presence of a secondary oxidant. A water soluble hypohalite compound is a preferred secondary oxidant. Since the nitroxide is not irreversibly consumed in the oxidation reaction only a small amount of it is required. During the course of the reaction it is the secondary oxidant which will be depleted. The amount of nitroxide required is in the range of about 0.005% to 1.0% based on cellulose present, preferably about 0.02-0.25%. The nitroxide is known to preferentially oxidize the primary hydroxyl located on C-6 of the anhydroglucose moiety of cellulose. It can be assumed that a similar oxidation will occur at primary alcohol groups on hemicellulose.
The nitroxide may first be premixed with a portion of an aqueous hypohalite to form a homogeneous solution before addition to the cellulose fiber slurry. Ultrasonic agitation may be useful to increase dissolution rate. The oxidation reaction may be allowed to continue over a time period from about one minute to ten or more hours at temperatures from about 0-75xc2x0 C., preferably about 0xc2x0-30xc2x0 C. Following the oxidation reaction, if maximum D.P. stability is desired, the cellulose is washed and reslurried in water where it is subjected to the action of a stabilizing reagent to convert substituent groups, such as aldehydes and ketones, to hydroxyl or carboxyl groups. Unstabilized nitroxide oxidized pulps have objectionable color reversion and will self crosslink upon drying, thereby reducing their ability to redisperse and to form strong bonds when used in sheeted products.
A preferred hypohalite is sodium hypochlorite (NaOCl). Sodium hypochlorite is inexpensive and readily available as a stable aqueous solution with about 4-10% NaOCl w/v. This can be made in situ by bubbling chlorine gas into a solution of NaOH. Admixture of NaOCl with sodium bromide (NaBr) will accelerate the oxidation reaction and the use of this combination is highly preferred. About 3 parts by weight NaBr to 4 parts of NaOCl has proved very satisfactory, although this ratio is not critical. The usage of NaOCl may be in the range of about 0.8-6.5 g/L of pulp slurry, preferably about 1.1-1.4 g/L. Usage of NaOCl based on cellulose will be within the range of about 0.5-35% by weight, preferably about 1.3-10.5% by weight. Exact usage will depend on the amount of carboxylation desired. The pH during oxidation should generally be maintained within the range of 8-11, preferably 9-10 and most preferably 9.5-9.8. The oxidation reaction will proceed at higher and lower pH values but at lower efficiencies.
A proprietary composition sold as Stabrex(trademark), available from Nalco Chemical Co., Chicago, Ill., may be used in place of the hypochlorite oxidant. Stabrex is sold as an aqueous stabilized highly alkaline solution of a bromine-containing composition having 1-5% NaOH, a minimum pH of 13, and is a latent source of hypobromite. The composition contains a stabilizer which is believed to be a sulfonated nitrogen containing compound. The Stabrex is useful where environmental or other considerations might dictate against the use of chlorine based materials.
It will be understood that in accordance with usual reaction kinetics the oxidation will proceed at a higher rate with increased concentrations of oxidants and at higher temperatures. Reaction at lower temperatures; e.g., at 0xc2x0-10xc2x0 C., is preferred from the standpoint of reducing cellulose D.P. degradation. However, the reaction may also be carried out at higher temperatures to produce products having a D.P. higher than 600.
Following oxidation, the cellulose is washed to remove any residual chemicals and may then be dried or further processed. If maximum stability and D.P. retention is desired the oxidized product is reslurried in water for treatment with a stabilizing agent. The stabilizing agent may either be a reducing agent or another oxidizing agent. A preferred reducing agent is preferably an alkali metal borohydride. Sodium borohydride (NaBH4) is preferred from the standpoint of cost and availability. However, other borohydrides such as LiBH4, or alkali metal cyanoborohydrides such as NaBH3CN are also suitable. NaBH4 may be mixed with LiCl to form a very useful reducing agent. When NaBH4 is used for reduction, it should be present in an amount between about 0.1 and 100 g/L. A more preferred amount would be about 0.25-5 g/L and a most preferred amount from about 0.5-2.0 g/L. Based on cellulose the amount of reducing agent should be in the range of about 0.1% to 4% by weight, preferably about 1-3%. Reduction may be carried out at room or higher temperature for a time between 10 minutes and 10 hours, preferably about 30 minutes to 2 hours.
Alkali metal chlorites are preferred oxidizing agents used as stabilizers, sodium chlorite being preferred because of the cost factor. Other compounds that may serve equally well as oxidizers are permanganates, chromic acid, bromine, and silver oxide. A combination of chlorine dioxide and hydrogen peroxide is also a suitable oxidizer when used at the pH range designated for sodium chlorite. Oxidation using sodium chlorite may be carried out at a pH in the range of about 1.5-5, preferably 2-4, at temperatures between about 25xc2x0-90xc2x0 C. for times from about 5 minutes to 50 hours, preferably about 10 minutes to 2 hours. One factor that favors oxidants as opposed to reducing agents is that aldehyde groups on the oxidized cellulose are converted to additional carboxyl groups, thus resulting in a more highly carboxylated product. These stabilizing oxidizers are referred to as xe2x80x9ctertiary oxidizersxe2x80x9d to distinguish them from the nitroxide/hypochlorite primary/secondary oxidizers. The tertiary oxidizer is used in a molar ratio of about 1.0-15 times the presumed aldehyde content of the oxidized cellulose, preferably about 5-10 times. In a more convenient way of measuring the required tertiary oxidizer needed, the preferred sodium chlorite usage should fall within about 0.001 g sodium chlorite/g of fiber to 0.2 g/g, preferably 0.01-0.09 g/g, the chlorite being calculated on a 100% active material basis.
After stabilization is completed, the cellulose is again washed and may be dried if desired. Alternatively, the carboxyl substituents may be converted to other cationic forms beside hydrogen or sodium; e.g., calcium, magnesium, or ammonium.
One particular advantage of the process is that all reactions are carried out in an aqueous medium to yield a product in which the carboxylation is primarily located on the fiber surface. This conveys highly advantageous properties for paper-making. The product of the invention will have at least about 20% of the total carboxyl content on the fiber surface. This is in comparison with about 10% as is the case with untreated fiber.
The carboxylated fiber of the invention is highly advantageous as a papermaking furnish, either by itself or in conjunction with conventional fiber. It may be used in amounts from 0.5-100% of the papermaking furnish. The carboxylated fiber is especially useful in admixture with recycled fiber to add strength. Its increased number of anionic sites should serve to ionically hold significantly larger amounts of cationic papermaking additives than untreated fiber. These additives may be wet strength resins, sizing chemical emulsions, filler and pigment retention aids, charged filler particles, dyes and the like. Carboxylated pulps do not hornify (or irreversibly collapse) as much on drying and are a superior material when recycled. They swell more on rewetting, take less energy to refine, and give higher sheet strength.
It is thus an object of the invention to provide a method of making a cellulose fiber having enhanced carboxyl content using an aqueous reaction medium.
It is also an object to provide a cellulose papermaking fiber having enhanced carboxyl content.
It is a further object to provide a cellulose fiber having an enhanced carboxyl content at the fiber surface.
It is another object to provide a carboxylated cellulose fiber that is stable against D.P. loss in alkaline environments.
It is yet an object to provide a stable cellulose fiber of enhanced carboxyl content with a D.P. of at least 850 measured as a sodium salt or 700 when measured in the free acid form.
It is still an object to provide a cellulose fiber having a high ionic attracion to cationic papermaking additives.
It is an additional object to provide cellulose pulp and paper products containing the carboxyl enhanced fiber.
These and many other objects will become readily apparent upon reading the following detailed description taken in conjunction with the drawings